This invention relates generally to charge-coupled devices, and, more particularly, to charge-coupled devices of the type used for processing analog signals.
Charge-coupled devices, usually referred to as CCD's, are a specialized form of large-scale integrated circuits that operate by movement and manipulation of units of electrical charge, rather than units of voltage or current, as in conventional integrated circuits and in most other electronic circuits. A gate in a charge-coupled-device consists of an electrode, an insulating layer, usually of silicon dioxide, and a substrate, usually of silicon. If a bias voltage is applied between the CCD gate electrode and the substrate, this creates a "potential well" in a depletion region under the electrode. Then, if minority carriers, either electrons or holes, depending on the doping polarity of the substrate, appear in the silicon substrate near the well, a charge packet will form in the depletion region. If a second, similarly structured gate is disposed adjacent to the first gate, and if the electrode bias voltage on this second gate is made higher than the bias on the first electrode, the packet of charge in the first well will be transferred to the second well.
In analog applications of CCD's, the magnitude of the packet of charge is made directly proportional to the amplitude of the signal being processed. In many analog applications of CCD's, analog data is sampled at a relatively high frequency, such as in video signal processing systems. Typical CCD structures have the form of delay lines or shift registers, in which phased clock signals are used to control the movement of charge packets along one or more strings or channels of CCD gates. Electrical input to such a channel can be effected by means of a p-n junction adjacent to the first gate. If a forward bias signal is applied to this diode, a large number of minority carriers will be made available, and a packet of charge will appear under the first gate. Output signals can be derived by means of a similar p-n diode and a series resistor, provided that the packets of charge are large enough. Otherwise, a more elaborate circuit, possibly utilizing field effect transistors, can be utilized.
When charge-coupled devices are used for relatively low-level analog signal processing, two major problems encountered are the occurrence of input circuit noise and the occurrence of output circuit noise. These two noise sources are so dominant that all other noise contributions are generally negligible. The input referred noise voltage, e.sub.n, is of the form e.sub.n =KT/C.sub.in, where K is the Boltzmann constant, T is the absolute temperature, and C.sub.in is the input capacitance of the circuit. This expression leads to a requirement for very large capacitors to achieve desirably low noise levels required by many systems, but the inherent capacitance of typical CCD input circuits is relatively low. Moreover, a larger capacitance can be achieved only at the expense of having larger circuit elements and a correspondingly lower packing density. Accordingly, it is one of the objects of the present invention to provide a CCD structure in which both the input noise and output noise are substantially reduced. As will soon be appreciated, in achieving this objective the invention also provides other important advantages over conventional CCD configurations.